substr function of string in c++

In C++, the `substr` function is used to extract a substring of a string. It has two common uses:

1. `substr(pos, len)`: Extract a substring of length `len` starting from position `pos`.
   - `pos`: Specifies the starting position of the extracted substring, starting from 0.
   - `len`: Specifies the length of the extracted substring. If `len` is not specified, all characters from `pos` to the end of the string are extracted by default.

2. `substr(pos)`: Extract the substring starting from position `pos` to the end.

The following is sample code using the `substr` function:

#include <iostream>
#include <string>

int main() {
    std::string str = "Hello, World!";
    
    // 提取从位置 7 开始的子串
    std::string sub1 = str.substr(7);
    std::cout << "sub1: " << sub1 << std::endl;  // 输出 "World!"
    
    // 提取从位置 7 开始的长度为 5 的子串
    std::string sub2 = str.substr(7, 5);
    std::cout << "sub2: " << sub2 << std::endl;  // 输出 "World"
    
    return 0;
}

In the above example, we used the `substr` function of string to extract the substring at the specified position and print the result.

It should be noted that the `substr` function in C++ returns a new `std::string` object, not a reference or pointer to the original string.
 

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